In electronic and optical devices, it has recently been proposed to construct devices having higher functions by hybrid connections of a plurality of elements. In the case of a hybrid device made by electrically connecting elements having functions different from each other (e.g., an optical semiconductor element and an electronic circuit element) in particular, the number of electrode pads and their positions vary between the elements, whereby a multilayer wiring board such as an interposer board is utilized.
Patent Literature 1 discloses a multilayer wiring board formed by stacking a plurality of ceramic substrates. While the ceramic substrates have various advantages, for example, in that their coefficient of linear expansion is smaller than that of resin-based substrates and that they can be flattened by polishing and the like, and techniques for providing them with through-hole electrodes have already been established, they are hard to form narrow-pitch wiring patterns and thus are not suitable for smaller sizes.
Therefore, techniques for employing silicon substrates in multilayer wiring boards have been under study in view of their capability of forming narrow-pitch wiring patterns in addition to their low coefficient of linear expansion and high flatness. Patent Literatures 2 and 3 disclose silicon substrates in which a portion surrounded by a ring-shaped groove (trench) having a depth equal to their thickness serves as an electric passage part.